Spatial Light Modulators are key components in optical data processing systems and in projection displays. They may be used, for example, to record information from optical signals by converting the same into coherent light distributions. Conventionally, the photosensitive layer of a Spatial Light Modulator is made of either heterojunction or homojunction photodiodes or photoconductors. An electro-optic layer of a typical Spatial Light Modulator often consists of a liquid crystal layer utilized in the twisted nematic mode, birefringent mode, or dynamic scattering mode. A Spatial Light Modulator of this type is often referred to as a "Liquid Crystal Light Valve."
In order to obtain better performance characteristics by increasing the impedance of the photoactive layer of the Spatial Light Modulators, before the instant invention, multilayers of thin films are used. For example, a two layer thin film structure consisting of ZnS and CdS is disclosed in U.S. Pat. No. 3,732,429 as necessary to obtain a sufficiently high dark impedance, in order to obtain a good impedance match to the liquid crystal layer for good performance when activated with light. Another method of impedance matching is described in U.S. Pat. No. 3,824,002 where a heterojunction of CdS and CdTe is used as the photosensitive layer. Yet another method of obtaining a high impedance photoactive layer is given in U.S. Pat. No. 4,206,979 where a multilayer heterojunction photodiode consisting of alternating layers of n-CdS and p-CdS:Cu is employed.